Polyamide-imides are thermoplastic, high performance, engineering polymers finding use in many applications, such as adhesives, coatings, filled and unfilled molding compositions, fibers, films, composites, laminates, etc., owing to a desirable combination of mechanical strength, heat resistance, chemical inertness and other properties. Torlon.RTM.polyamide-imides, available from Amoco Performance Products, Inc., of Ridgefield, Conn., are examples of commercial polyamide-imides.
It is known that volatiles, such as water and carbon dioxide, generated due to condensation, imidization and other reactions that occur in preparation, molding or other high temperature processing of polyimides or polyamide-imides, including their polyamide-amic acid precursors, or water vapor given off during such processing due to moisture absorbed by such polymers, can cause foaming. As a result, while seldom desired or intentionally produced, polyamide-imides of cellular structure have long been known and observed in various stages of polyamide-imide preparation and processing.
For example, it has been known for several years that polyamide-imides can foam when purged from the nozzle of the injection barrel of an injection molding machine. Published technical information related to Torlon.RTM.engineering resins, Amoco Chemicals Corporation Bulletin TAT-la, discloses that in setting up for injection molding of such materials, purge shots should be examined for excessive foaming and that if the same occurs the material should be redried for 8-24 hours at 350.degree. F. Injection molded parts with a cellular structure also can result when the amount of polymer injected into a mold is insufficient to fill it or, as disclosed in the above-mentioned Bulletin TAT-la, when mold closed time is too short. It also is well known that polyamide-imide parts prepared by extrusion may have voids.
While neither intended nor particularly desirable, it also is known that commercially available polyamide-imide pellets, generally having lengths and diameters up to about one-half inch (about 1.3 cm), can sometimes exhibit irregular cellular structures and densities below those of solid polymer. Pellets usually are prepared by plasticating polymer flake or powder using a screw conveyor, extruding the plasticated polymer through a strand die, continually removing strands of extrudate from the die, cooling the strands and chopping the cooled strands into pellets. When the softened extrudate exits the die, expansion of the strand due to volatiles liberated during plastication may result in formation of a cellular structure unless the strand is cooled sufficiently rapidly after exiting the die to prevent expansion. Of course, it is desirable to maximize pellet density to maximize throughput of resin and minimize packaging, shipping and other downstream handling and processing of gas(es) present in any cellular material. Accordingly, such efforts as are intentionally directed to controlling density in such pelletizing operations aim to maximize density and, thus, minimize foaming.
Commonly assigned U.S. Pat. No. 4,581,264 to Emery et al., issued Apr. 8, 1986, teaches that in production of articles of manufacture by extrusion of compositions comprising polyamide-imide, introducing air and moisture into the extrusion system is undesirable because articles that are foamed or have porosity can result. Emery et al. also teaches that use of a single-screw extruder is undesirable because parts having porosity can result.
Other examples of polyamide-imides having foamed or cellular structures are described in commonly assigned U.S. Pat. No. 4,167,620 to Chen, issued Sept. 11, 1979, directed to heat treating shaped articles comprising polyamide-imide to avoid foaming, distortion and sacrifices in properties resulting from thermal shock. Without such treatment, thermally shocked articles exhibit irregularly sized voids or cells, often distributed randomly throughout the articles. Blisters may be present on the surfaces of such articles. Physical properties generally are poor and frequently nonuniform throughout the articles. Chen is directed to preventing void formation and other deleterious results of thermal shock by heat treating polyamide-imide shaped articles to drive off volatiles without substantial distortion, formation of voids and other adverse effects. Commonly assigned U.S. Pat. No. 4,403,061 to Brooks et al., issued Sept. 6, 1983, avoids such undesirable effects, while reducing requirements for heat treatment, by adding to polyamide-imides metal oxides capable of tying up water that otherwise might contribute to foaming, distortion and other sacrifices.
As is known, polyamide-imide foams obtained as described above exhibit irregular, open-cell structure. Interconnected, randomly sized and shaped voids often vary in size, shape and distribution from one area of a foam to another. Such nonuniformity is hardly surprising given that foaming usually takes place only inadvertently, with efforts related to foaming being directed toward avoiding, rather than promoting, the same.
Polyamide-imide foams also have been produced intentionally. For example, foamed rods and boards of Torlon.RTM.polyamide-imide prepared by extrusion using circular and sheet dies, respectively, and without rapid cooling of extrudate but otherwise similar to the above-described pelletizing procedure exhibit useful cell structure and other properties although control over foaming is limited. Satisfactory foam structure and properties also have been obtained by heating commercial Torlon.RTM.polyamide-imide powder or pellets in a closed mold to above glass transition temperatures ("Tg") of the polymers, generally at about 65.degree. to about 200.degree. C. above Tg, for a time sufficient to soften and fuse the particles and cause absorbed and evolved volatiles to expand the polymer to fill the mold. Residence times in the mold can range from several minutes to several hours, with time and temperature being adjusted to achieve suitable softening, fusion and expansion without charring the surface of the final product. After heating, the mold is cooled to below Tg whereby the expanded cellular mass solidifies and a foamed shape conforming to the mold cavity is removed. Foaming of polyamide-imides by such a technique, sometimes referred to as confined free rise or confined free expansion foaming, is employed for fabrication of shaped articles of simple geometry such as panels, sheets and blocks.
Foamed polyamide-imides prepared by extrusion and confined free rise as described above have densities generally ranging from about 15 to about 50% of those of the unfoamed, starting polyamide-imides. In the case of Torlon.RTM.polyamide-imides having unfoamed densities of about 1.4 g/cm.sup.3, foam densities of about 0.2 to about 0.7 g/cm.sup.3 (about 15-40 lb/ft.sup.3) are commonly achieved by confined free rise foaming. The foams have a substantially open-cell structure of interconnected cells, generally of about 0.05 to about 0.5 inch (about 0.12-1.3 cm) diameter, compressive strengths of about 250 to about 800 psi (about 17-56 kg/cm.sup.2), and K factor, indicative of thermal insulation and decreasing with decreasing density, of about 0.4 to about 1.2 btu in/ft.sup.2 hr .degree. F. (about 1.38 .times.10.sup.-4 -4.13 .times.10 .sup.4 cal/sec cm .degree. C.). The foams char when exposed to flame, extinguishing on removal thereof. Screw fastener retention is sufficient to permit attachment with screws to or between other materials, e.g., metal, wood, concrete or foamed or unfoamed synthetic materials, or in a sandwich-type structure having such other materials between layers comprising polyamide-imide foam.
While such intentionally foamed polyamide-imides can exhibit useful properties, preparation thereof as described above suffers from poor control over foaming in the case of extrusion and is time-consuming and limited by mold and oven size and to shapes of simple geometry in the case of confined free rise foaming. Further, for a given polymer composition to be foamed by heating in a closed mold, variations in particle size of the particulate polymer placed in the mold can have a significant effect on process conditions required to achieve useful properties or on properties themselves. Larger particles retard diffusion of volatiles out of the mass of particles as it is heated to soften and fuse the same; if heating is too severe, for example, at too high a temperature or for too long a time, formation of char or large, irregular voids can result. Conversely, with smaller particles, heating at too low a temperature or too slowly can result in insufficient foaming due to loss of volatiles by diffusion prior to softening and fusing of the polymer.
Polyamide-imide foams also have been prepared intentionally by so-called short-shot injection techniques in which an amount of softened, fused polymer effective to yield a desired density without filling a mold is injected into the mold. Polymer in powdered or pellet form is introduced into a hopper and conveyed by a screw conveyor through a heated barrel of an injection press or like apparatus. As the polymer proceeds along the length of the barrel it softens, the softened particles fuse and volatiles are generated. Foaming of polymer in the barrel is substantially prevented by maintaining suitable back pressure within the barrel. A shut-off nozzle is used to maintain a melt seal in the barrel, preventing undesirable nozzle foaming prior to injection. From the exit end of the barrel the polymer and volatiles are forced into the mold, the amount injected and, in turn, density of the foam being prepared, being controlled by the length of the injection stroke. An example of a suitable injection press is a Natco 500 ton injection molding machine available from Natco Corporation. After rapid expansion of the foamable composition in the mold, the resulting expanded composition is cooled to below Tg and a foam with an outer skin of solid, unfoamed or essentially solid polyamide-imide is removed from the mold. Although more complex shapes can be prepared by short shot injection foaming than by confined free rise expansion, the former is expensive and limited to parts of higher densities than in confined free rise unless carried out with a heated mold which adds additional expense.
Polyamide-imide foams prepared by short shot injection foaming, like those prepared by confined free expansion, are of open cell structure, generally with cells of similar size and shape. Torlon.RTM.polyamide-imide foams prepared by short shot injection foaming typically have densities of about 0.5 to 0.6 g/cm.sup.3.
While not directed to solving problems encountered in such prior art methods, the following patents may be of interest in connection with this invention in disclosing various foams prepared by techniques in which volatiles generated during processing contribute to foaming
U.S. Pat. No. 4,394,664 to Gagliani et al., issued July 19, 1983, discloses foams prepared by reacting pyromellitic dianhydride or 3,3',4,4'-benzophenone tetracarboxylic acid dianhydride and certain oxoimines, esterifying the result, reacting the esterification product with diamine, drying the result to a free-flowing powder or flake and then heating to imidize and condense with liberation of alcohol and water which expand the polymer to a self-supporting cellular structure. Preparation of foamed sheets by heating such polymers in an oven is disclosed in the examples. U.S. Pat. No. 4,426,463 to Gagliani et al., issued Jan. 17, 1984, discloses foams of increased compressive strength prepared similarly but at reduced oxoimine-to-dianhydride ratios and U.S. Pat. No. 4,539,336 to Long et al., issued Sept. 3, 1985, discloses a related preparation in which dianhydride and oxoimine are reacted in alcohol.
From commonly assigned U.S. Pat. No. 3,300,420 to Frey, issued Jan. 24, 1967, it is known to prepare foamed plastics or dense molding resins, depending on reaction conditions, by reaction of trimellitic anhydride and aryl polyisocyanates with liberation of carbon dioxide. According to the patent, carrying the reaction to completion with minimum confinement of polymer results in foam of excellent cell structure and physical properties.
U.S. Pat. No. 3,483,144 to Lavin et al., issued Dec. 9, 1969, discloses in situ preparation of polyimide foams by mixing tetracarboxylic acid and polyamine to form a system with volatiles content of at least 6.2% and heating to simultaneously polymerize and foam. Foaming is said to occur due to volatilization of solvent used in preparation and water vapor evolved from condensation reactions.
U.S. Pat. No. 3,926,911 issued Dec. 16, 1975, and U. S. Pat. No. 3,948,835 issued Apr. 6, 1976, both to Greber et al., disclose silicon-modified polyamides, polyamide-imides or polyimides prepared from polycarboxylic acid compounds and aminosilanes. Use of the polymers to form foams and heating of prepolymer powder in a closed mold to produce hard foam of uniform pore size are disclosed.
While the foregoing disclose certain foams and that residual solvents or reaction by-products, such as water vapor, cause or contribute to foaming, the disclosed foams are prepared in situ during polymerization or by a confined free-rise technique as described above, i.e., by heating in a closed mold. These patents do not address the problems of long processing times, part size/shape limitations, particle size effects or process complexity and expense.
Also of possible interest in connection with this invention is Modern Plastics Encyclopedia, Vol. 54 No. 10A (1977-1978), pp. 294-302, discussing generally various types of conventional thermoplastic foams and production thereof, including profile extrusion with vacuum sizers or sizing dies and structural foam molding. Foaming of polyamide-imides and problems associated with control of foaming of such resins due to volatiles evolved during heating of the same are not addressed.
As can be appreciated, there is a need for a process for manufacture of polyamide-imide foams having useful properties at greater production rates than previously achieved with greater flexibility as to part size.
It is an object of this invention to provide such a process. A further object is to provide a continuous process for preparing polyamide-imide foams. A further object is to provide a method for producing high strength, foam comprising polyamide-imide having utility in applications such as automotive or aerospace insulation materials and firewalls. A still further object is to provide for production of polyamide-imide foam parts with flexibility as to part size and geometry and at greater production rates with better control over foaming than offered by known techniques.
We have found that the objects of this invention can be achieved by extruding a foamable composition comprising softened, fused polyamide-imide and volatiles into a confined expansion zone, expanding the foamable composition in the confined expansion zone and cooling the expanded composition. Advantageously, foam density is easily controlled and foam parts of large or small size can be prepared. Further, the process can be conducted continuously, and with good control over foaming, thereby offering advantages over known techniques in terms of process speed and reliability. Further, particle size effects encountered in confined free-rise foaming are eliminated in the invented process by softening and fusing a foamable composition comprising polyamide-imide and volatiles before introduction of the composition into a confined expansion zone. The invented process is more flexible as to part size and density than short shot injection foaming. Further, other things being equal, the invented process offers advantages as to production rates because parts are prepared by an extrusion process which allows for production of parts that are continuous in one dimension, as opposed to the closed molds used in both short shot injection foaming and confined free expansion. Foams comprising polyamide-imide prepared according to the present invention, like those prepared by known techniques, exhibit a desirable combination of density, structural strength, insulating and other properties. The foams have a substantially open cell structure with cell sizes and shapes being comparable to those of known, intentionally prepared polyamide-imide foams. As prepared, the foams usually are encased within essentially continuous polyamide-imide skins, the densities of which equal or approach those of the solid, unfoamed polyamide-imide composition used in the invented process at the surface of the foamed part and decrease toward its interior. Such skins, while providing extra strength, can be removed easily if desired. The skins may have utility as regrind polymer in the invented process.